Carbonization of cellulosic fibrous materials in the presence of an organosilicon compound

ABSTRACT

The subject of the present invention is a method of obtaining fibrous carbon materials by carbonization of cellulosic fibrous materials carried out continuously or batchwise in the presence of at least one organosilicon compound. Characteristically, said organosilicon compound is chosen from the family of cyclic, linear or branched polyhydrosiloxanes which are substituted with methyl and/or phenyl groups and the number-average molecular mass of which is between 250 and 10,000, advantageously between 2,500 and 5,000.

[0001] The subject of the present invention is a method of carbonizingcellulosic fibrous materials in the presence of at least oneorganosilicon compound for the purpose of obtaining fibrous carbonmaterials. Said carbonization may be carried out both continuously andbatchwise. The fibrous carbon materials obtained may then be heattreated (especially graphitized) in order to generate fibers having thedesired properties.

[0002] Carbon fibers having a cellulosic precursor were the first carbonfibers manufactured in the world. Starting from such cellulosicprecursors, Edison, at the end of the last century, obtained filamentsfor his incandescent lamps (U.S. Pat. No. 223,898).

[0003] However, polyacrylonitrile has proved for a long time to be amore suitable precursor for obtaining high-strength high-modulus carbonfibers, more particularly those intended for the reinforcement ofcomposites.

[0004] However, carbon fibers from viscose have been used since 1955 inthe manufacture of carbon/phenolic resin composites employed as thermalprotection for propulsion units. These low-modulus fibers have arestricted thermal conductivity. Manufacturing these fibers requires aparticular rayon-type precursor: a rayon having a disorientedcrystalline texture (R. Bacon, “Carbon Fibers from Rayon Precursors” inChemistry and Physics of Carbon, 1973, Vol. 2, Marcel Dekker, New Yorkand P. Olry, 14th Biennial Conference on Carbon, 1979).

[0005] More recently, it has proved possible to carbonize, with usefulresults, rayons of another type, especially highly oriented rayons,thanks to the intervention of an organic silicon derivative duringcarbonization.

[0006] Thus, continuous carbonization of unidirectional fabrics or websof cellulose fibers has proved possible and resulted in carbon fabricsor carbon yarns, having a strength appreciably better than that offabrics or yarns obtained by the conventional method (which comprisesbatchwise precarbonization followed by continuous carbonization), oncondition that said fabric or said web be preimpregnated with a fewpercent of an organosilicon product. This has been described inparticular in Russian patents RU 2045472 and 2047674.

[0007] Said organosilicon product was disclosed therein as an oligomerchosen from polydimethyl phenylallylsilanes, polysiloxanes,polymethylsiloxanes, polysilazanes and polyaluminoorganosiloxanes. Infact, its precise nature is not really specified.

[0008] Said patent RU 2047674 also discloses the advantage of makinguse, on the cellulosic substrates to be carbonized, apart from saidorganosilicon product, of a mineral additive called a “fire-retardingcompound”, such as NH₄Cl.

[0009] In this context, the novelty of the present invention lies in theselection of specific organosilicon compounds—additives for thecarbonization of cellulosic fibrous materials—which are particularlyeffective. Said compounds have proven to be very efficient for improvingthe properties of the carbon fibers obtained from the carbonization,this being so with any type of carbonized cellulosic material(especially commercial staple fibers and rayons), whether saidcarbonization is carried out continuously or batchwise. Nevertheless,although the use of said compounds constitutes an undeniable benefitwhen carrying out carbonizations batchwise and continuously, it provesto be indispensable for the continuous carbonization of certainsubstrates (it makes said continuous carbonization of said substratespossible). The present invention therefore relates to the use of oneparticular family of organosilicon compounds within said context.

[0010] The subject of the present invention is in fact a method ofobtaining fibrous carbon materials by carbonization of cellulosicfibrous materials carried out continuously or batchwise in the presenceof at least one organosilicon compound. Characteristically, saidorganosilicon compound is chosen from the family of cyclic, linear orbranched polyhydrosiloxanes, which are substituted with methyl and/orphenyl groups and the number-average molecular mass of which is between250 and 10,000, advantageously between 2,500 and 5,000.

[0011] It is assumed that the increase in the strength of the filamentsduring carbonization in the presence of such additives, compared withthat of filaments carbonized without an additive, is due to the bridgingof the carbon chains during aromatization by said additives and/or theirtransformation products. This reinforcement of the carbon network takesplace only at the surface of the fibers, but the reduction in surfacedefects which results therefrom causes a substantial increase in thestrength of the filaments.

[0012] The magnitude of this reinforcement, with the additives of theinvention, is remarkable. It makes it possible to counteract theshrinkage during carbonization and even to stretch the fibers (up to50%) without them breaking, thereby ensuring orientation of the textureof said fibers and a reduction in or rearrangement of the internalpores. It has made it possible to obtain, with any type of cellulose(solvent celluloses and rayons, especially for tires), filaments whichhave strengths of around 1,500 to 2,000 MPa and moduli of around 70 to110 GPa.

[0013] According to the invention, the family of additives used is thatof polyhydrosiloxane oligomers (oligomers, because of theirnumber-average molecular mass, of between 250 and 10,000, generallybetween 250 and 7,000, advantageously between 2,500 and 5,000). Sucholigomers:

[0014] owing to their molecular mass and the presence of methyl and/orphenyl groups in their chemical structure, have suitable thermalstability (optimized with respect to that of cellulose, i.e. they arestable up to the degradation temperature of cellulose and capable ofreacting with the degradation products of said cellulose at thedegradation temperature of the latter) and a viscosity compatible withtheir use;

[0015] owing to the presence of —SiH reactive functional groups in theirchemical structure, are capable of reacting via hydrosilylationreactions with the cellulose degradation products, within which productsdouble bonds are formed.

[0016] The subject matter developed in the above paragraph, which seemsvery logically to explain the very good results obtained with theadditives of this family, was obviously developed a posteriori.

[0017] Polyhydrosiloxanes of said family are commercially available atthe present time. Certain polyhydrosiloxanes are for example sold byRhodia Silicones.

[0018] Advantageously, these polyhydrosiloxanes are used prior tocarbonization, the fibrous cellulosic materials being pre-impregnatedwith them. To carry out such an impregnation, said polyhydrosiloxanesare generally used dissolved in a solvent, such as perchloroethylene.Such a solvent can easily be removed before carbonization.

[0019] It may be pointed out here, in general, that saidpolyhydrosiloxanes selected according to the invention are used, ofcourse, in an effective amount, generally from 1 to 10% by weight, withrespect to the weight of cellulosic materials. They have to be used insufficient quantity to observe the expected effect, but not in excessivequantity as then an inopportune bonding effect may be observed. A personskilled in the art is able to optimize the amount of organosiliconcompounds to be used, the use of which is recommended within the contextof the method of the invention.

[0020] The inventors have also noted that the beneficial effect of saidorganosilicon compounds could be further enhanced by the combined use ofa mineral additive.

[0021] According to a preferred variant for implementing the method ofthe invention, the cellulosic fibrous materials are thus alsoimpregnated, before they are carbonized, with at least one mineraladditive, a Lewis acid or base.

[0022] Said mineral additive may especially be chosen from ammonium andsodium halides, sulfates and phosphates, urea and mixtures thereof.

[0023] Advantageously, it consists of ammonium chloride (NH₄Cl) ordiammonium phosphate [(NH₄)₂HPO₄].

[0024] The method may also involve two successive impregnations of thecellulosic fibrous material to be carbonized (one with an organosiliconcompound and the other with a mineral additive, in any order).

[0025] When such a mineral additive is used, it is possible to obtainvery promising results, especially high strengths in the case of carbonfibers and to do so with a better carbon yield (from 25 to 30%) thanthat obtained without said additive (from 15 to 20%).

[0026] As already indicated, the additives of the invention areadvantageously used, both in carbonization processes carried outbatchwise and in carbonization processes carried out continuously. Ithas been seen that they make it possible to carry out certaincarbonizations continuously (which carbonizations were, according to theprior art, only possible to carry out batchwise).

[0027] According to a preferred implementation variant, the method ofthe invention is thus carried out continuously.

[0028] Finally, it will be recalled that the method of theinvention-carbonization of cellulosic fibrous materials in the presenceof specific organosilicon compounds-is particularly beneficial in thatit allows effective carbonization, batchwise and continuously, of anytype of cellulose, packaged in various forms.

[0029] The cellulosic fibrous material may especially be in the form oftextile yarns or surfaces (wovens, knits, felts, nonwovens,unidirectional webs, unidirectional tapes, . . . ).

[0030] Said cellulosic fibrous material may especially consist of anytype of rayon or staple fiber. The method of the invention is, in thiscase, particularly beneficial: it results, used with products widelyavailable on the market, in high-quality fibrous carbon materials.According to the prior art, such high-quality materials could beobtained only from cellulosic fibrous materials of a very particulartype.

[0031] It is therefore recommended to implement the method of theinvention—the use of the organosilicon compounds described above—in thecarbonization of such cellulosic substrates, widely available on themarket, such as the rayons intended hitherto for reinforcing tires.

[0032] Of course, the field of application of said method is not limitedto the carbonization of these substrates . . .

[0033] The invention will now be illustrated by the examples below.

EXAMPLE 1

[0034] A 3,680 dtex high-tenacity cellulose yarn (super 3 type), havinga strength of 50 cN/tex (12.7 μm filament diameter), was desized byperchloroethylene then impregnated with 2.5% by weight of apolyhydrophenylmethylsiloxane having a viscosity of 10 Pa.s, containing90% —Si(CH₃)₂—groups, 5% —Si(CH₃) (C₆H₅)—groups and 5% —Si(CH₃)H—groupsin addition to —Si(CH₃)₂H of the chain ends, having a number-averagemolecular mass of 3,850, by passing it through a 3 wt % solution of thispolyhydrosiloxane in perchloroethylene. After removing theperchloro-ethylene, the cellulose yarn was pyrolyzed in static mode, atup to 1,200° C., so as to shrink freely according to the thermal profilebelow:

[0035] at 170° C., for 90 min, in air;

[0036] in succession: 230° C., 285° C., 315° C., 330° C., 400° C., 485°C., 555° C. and 655° C., for 5 min at each of these temperatures, innitrogen;

[0037] at 1,200° C., for 2.5 min, in nitrogen.

[0038] This pyrolysis, carried out batchwise within the context of theexample, could just as well have been carried out continuously.

[0039] The carbon filaments extracted from the carbonized yarn had atensile strength of 1,125 MPa and a modulus of 40 GPa for a diameter of5.8 μm. The carbonization shrinkage along the axis of the fibers was40%.

[0040] The carbonization yield was 15.6%.

EXAMPLE 2

[0041] A cellulose yarn identical to that of example 1 was desized withperchloroethylene, then impregnated with 2.5 wt % of apolyhydromethylsyloxane resin sold by Rhodia Silicones (with thereference: RHODORSIL RTV 141 B) by dipping it in a 3 wt % solution ofthe product in perchloroethylene. The pyrolosis was carried out, withfree shrinkage, according to the thermal profile of example 1.

[0042] The carbon filaments extracted from the yarn had a tensilestrength of 1,100 MPa, a modulus of 40 GPa and a diameter of 5.7 μm. Thecarbonization shrinkage along the axis of the fibers was 40%.

[0043] The carbonization yield was 15.2%

EXAMPLE 3

[0044] A cellulose yarn identical to that of example 1 was desized andthen impregnated with the organosilicon additive as in example 1. It wasthen impregnated with 8% by weight of NH₄Cl by passing it through a 13wt % aqueous solution of said NH₄Cl.

[0045] The yarn was dried at 100° C. for 30 min and the excess NH₄Cl wasremoved by rinsing for a few seconds in distilled water.

[0046] Said yarn was dried at 100° C. for 1 hour and then underwentpyrolysis at 1,200° C. as in example 1.

[0047] The tensile strength of the carbon filaments extracted from saidcarbonized yarn was 1,200 MPa and their modulus was 45 GPa, for adiameter of 8.3 μm. The shrinkage during carbonization was 32.3%.

[0048] The carbonization yield was 30%.

EXAMPLE 4 (Comparative Example)

[0049] A cellulose yarn identical to that of example 1 was desized withperchloroethylene and then, without being impregnated with thepolyhydrosiloxane additive, it was pyrolyzed according to the thermalprofile indicated in said example 1.

[0050] The tensile strength of the carbon filaments extracted from theyarns obtained was only 660 MPa and their modulus was 38 GPa. Thediameter of said filaments was 5.8 μm.

1. A method of obtaining fibrous carbon materials by carbonization ofcellulosic fibrous materials carried out continuously or batchwise inthe presence of at least one organosilicon compound, characterized inthat said organosilicon compound is chosen from the family of cyclic,linear or branched polyhydrosiloxanes which are substituted with methyland/or phenyl groups and the number-average molecular mass of which isbetween 250 and 10,000, advantageously between 2,500 and 5,000.
 2. Themethod as claimed in claim 1, characterized in that, prior to saidcarbonization, said cellulosic fibrous materials are impregnated withsaid organosilicon compound.
 3. The method as claimed in either ofclaims 1 and 2, characterized in that, prior to said carbonization, saidcellulosic fibrous materials are impregnated with at least one mineraladditive, a Lewis acid or base.
 4. The method as claimed in claim 3,characterized in that said mineral additive is chosen from ammonium andsodium halides, sulfates and phosphates, urea and mixtures thereof andadvantageously consists of ammonium chloride (NH₄Cl) or diammoniumphosphate [(NH₄)₂HPO₄].
 5. The method as claimed in any one of claims 1to 4, characterized in that said carbonization is carried outcontinuously.
 6. The method as claimed in any one of claims 1 to 4,characterized in that said carbonization is carried out batchwise. 7.The method as claimed in any one of claims 1 to 6, characterized in thatsaid cellulosic fibrous materials consist of textile yarns or surfacesof the type comprising wovens, knits, felts, nonwovens, unidirectionalwebs or unidirectional tapes.
 8. The method as claimed in any one ofclaims 1 to 7, characterized in that said cellulosic fibrous materialsare products widely available on the market and especially rayonssuitable for reinforcing tires.